Electrolytic preparation of lanthanide and actinide hexaborides using a molten, cryolite-base electrolyte

ABSTRACT

Lanthanide and actinide hexaborides are electrolytically synthesized using a molten salt electrolyte containing a major portion of cryolite and minor portions of an alkali borate and a source compound to supply the lanthanide or actinide metal. Minor amounts of an alkali hydroxide or carbonate added to the electrolyte tend to improve product recovery and purity. The synthesis may be accomplished in a cell open to the atmosphere to produce highly pure lanthanide and actinide hexaborides.

United States Ratent 11 1 Gomes et al.

1451 Sept. 2, 1975 ELECTROLYTIC PREPARATION OF LANTHANIDE AND ACTINIDE HEXABORIDES USING A MOLTEN, CRYOLITE-BASE ELECTROLYTE [75] Inventors: John M. Gomes, Reno. New; Kenji Uchida, Sakura-mura, Japan [73] Assignee: The United States of America as represented by the Secretary of the Interior, Washington. DC.

[22] Filed: Oct. 4, I973 [21] Appl. No.: 403.600

52 u.s.c1 204/15; 204/86 51 int. Cl.- C2 5B 1/00 Field of Search 204/15,

[5 6] References Cited OTHER PUBLICATIONS Cueilleron et al., Chem. Abs, Vol. 67, 74972y, (1967).

Primur E,\'uniirzerLeland A Sebastian Attorney, Agent, or FirnzRoland H. Shubert; Donald R. Fraser 57 ABSTRACT 12 Claims, N0 Drawings ELECTROLYTIC PREPARATION OF LANTHANIDE ANI) AC'IINIDE IIEXABORIDI'IS USING A MOLTEN, CRYOIJ'IE-BASE ELECTROLYTE BACKGROUND OF THE INVENTION Lanthanide and actinide hexaborides generally display high melting points. extreme hardness. and interesting electrical properties. Lanthanum hexaboride. for example, is reported to possess the highest electronic emissivity of any known substance. As a group. these hexaborides are useful as base constituents for high temperature materials and in applications utilizing their thermionic emission properties as in thermionic power generation. I

Known methods of preparing lanthanide and actinide hexaborides include the following: l Direct combination of the metal with boron; (2) carbon reduction of mixtures of metal oxide and boron anhydrides; (3) reduction of metal oxide with boron carbide; (4) reduction of metal oxide with boron; and (5) fused salt electrolysis. Of these techniques. reduction of metal oxides with boron carbide is known to be practiced commercially but the process requires highly pure boron carbide and metal oxides. The product is often contaminated with boron and metal carbides.

Preparation of lanthanide and actinide hexaborides using known techniques are reported to codeposit boron as a by-product (Samsonov et al., Boron and Its Compounds and Alloys, Publishing House of the Academy of Sciences Ukrainian SSR Kiev, 1960, pp. 447497). This results in borides containing less than stoichiomctric quantities of metal. Codeposited boron cannot be removed by conventional solvent leaching. Many investigatorssuch as Andrieux (L. Ann. Chim. Phys., 12, 42, I929) preferred electrolytes containing a lanthanide oxide, alkaline earth oxide, alkaline earth fluoride, and boric oxide. A shortcoming of these electrolyte systemswas the difficulty in obtaining a singlephase product:

SUMMARY or THE INVENTION We prepared lanthanide and actinide hexaborides by passing a dc current through a molten electrolyte composed of alkali aluminum fluoride and alkali boratc. Dissolved' in the bath is an oxide, fluoride or chloride of a lanthanide or actinide metal. Minor amounts of an alkali hydroxide or carbonate added to the bath tends to improve recovery and product purity. Coarse. crystalline hexaboride product is deposited on the cathode. A protective atmosphere is unneccessary. Electrolysis temperatures may vary from about 900 to about I,l0OC and the process may be carried out in a batch, semicontinuous or continuous fashion.

Hence, it is an object of our invention to produce lanthanide and actinide hexaborides.

It is a specific object of our invention to synthesize lanthanide and actinide hexaboride using a cryoliteborate electrolyte in a cell open to the air.

DETAILED DESCRIPTION OF THE INVENTION Our process comprises. in its broadest sense, the passing of a direct current through a molten salt mixture containing a lanthanide or actinide compound to form a metal boride deposit on the cathode. The metal boride, which may be a lanthanide hexaboride, an actinide hexaboride, or mixtures of lanthanide and/or acti- 2 nide hexaborides. is deposited on the cathode as clusters ofdendritic crystals. Recovery of the metal boride product is accomplished by removing the cathode from the electrolyte and physically dislodging the adhering crystals. Separation of the metal boridc crystals from adhering electrolyte may be accomplished by leaching in hot, diluted sulfuric acid.

Constituents of the electrolyte preferably are of technical grade and, of course. are in the anhydrous form. Use of technical grade salt offers considerable economic advantage in carrying out our process while the use of more highly pureelectrolyte components contributes little if any improvement to the process.

The electrolyte constituents comprise a major portion of an alkali aluminum fluoride such as cryolite together with minor amounts of an alkali borate and a lanthanide or actinide compound. Additions of minor amounts of alkali hydroxides or carbonates to the electrolyte tend to increase product recovery and purity. The alkali horate preferably is a sodium borate and may be a metaborate, orthoborate, diborate, tetraborate, pentaborate, or mixtures thereof. Naturally occurring borate compounds such as borax may also be used. The lanthanide or actinide compound may be in the form of a chloride, fluoride, oxide, or mixtures thereof. Since a substantial degree of refining occurs during the electrodeposition, impure lanthanide or actinide oxide concentrates may advantageously be used in our process.

Composition of the electrolyte may vary over a fairly wide range. The following table illustrates a range of electrolyte compositions which have been found to be suited for use in our process:

Operating temperatures may range from about 900 to l,100C. but best results are obtained in the temperature range of 950 to l,05()C. This last range is prefcrred. Current density can vary widely; from about 15 to amps per square decimeter. Depending somewhat upon cell geometry, the cell potential is generally in the range of 2.0 to 5.0 volts.

The electrolytic cell may conveniently comprise a conductive crucible serving the double function of container and anode. Aconductive rod or plate, preferably centrally positioned, may serve as the cathode. Graphite is suitable for use in all portions of the electrolytic cell in contact with the electrolyte when potassium is absent from the electrolyte composition. Potassium ions intercalate with graphite resulting in physical damage or even failure of graphitic cell components. For this reason. as well as for reasons of economy, we prefer sodium as the alkali metal component of our electrolyte. Other refractories, such as silicon nitride, may also be used in the fabrication of electrolytic cells suitable for use in our process. Since the process does not require a protective atmosphere, any convenient heating means, including oil or gas fired furnaces, may be used to maintain the required electrolysis temperatures. A particularly preferred heating means consists of an electric resistance pot furnace.

3 When operating our process in a batch manner, the bath ingredients are mixed and fused. After fusion, which effectively dehydrates any salts in the hydrated or partially hydrated form. the bath is stabilized at op- 4 boride was 0.2g/amp-hr. Vacuum fusion and combustion analysis of the product indicated the carbon content to be 0.72% and the oxygen content to be 0.52%. A spectrographic analysis of the CeB product showed crating temperature. The cathode member is inserted the following contaminating metals in weight percent: into the bath and electrolysis is begun. Electrolysis is A], 0.5; Ba, 0.05; Ca, 0.10; Cu, 0.0l; Fe, 0.35; Mg continued for a period of time sufficient to build up an 0.01; Mn, 0.14; Si, 0.20, and Ti. 0.0l. adhering mass of boride crystals on the cathode after which the cathode is removed from the bath. Excess EXAMPLE 2 electrolyte is shaken from the cathode deposit and the H) The eletrolytic cell of Example 1 was used to synthedeposit is then physically removed from the cathode as size and deposit lanthanide hexaborides using an imby scraping. Remaining electrolyte is leached from the pure lanthanide oxide feed material of the following crystal mass using acids. Hot sulfuric acid diluted about composition: 1:] with water is a preferred leaching agent. After Table 3 leaching, free carbon can be removed by water elutriaor b heavy di Separation The process y b Analysis of lmpure Lanthanidc Oxides Feed Material operated on a scmicontinuous or continuous basis by Weight-Percent Weight-Percent adding electrolyte components to the bath as they be- Cc 47 2 Cu 0 3 come depleted and by periodically changing cathodes, 5: thus removing the boride product from the electrolytic 30 2 Cell- 5111 I; s :l The following examples represent specific embodi- Gd .1 Si |.0 ments of our process and serve to more fully illustrate Y F our invention:

EXAMPLE 1 A l000g electrolyte charge was prepared having the An electrolytic cell was constructed which consisted followmg composmon: of a graphite crucible functioning both as a container Table 4 and an anode and a graphite rod which served as a Ompmmm WCEMPUCCM Mowpcrccm cathode. The anode crucible was three inches in interv nal diameter and seven inches in height. while the catha hias 7 5 ode was 1 inch in diameter. The cathode was centrally 0 13 6 5 30 placed within the anode crucible leaving a l-inch space 22%;?" g between the cathode and sidewalls and l /2 inches from the bottom. Heating means for the cell consisted of an 35 electric resistance furnace and the cell was open to the An electrolysis. Consisting of three onc hour Cycles, was then performed at a temperature of l0O0 i 10C. A liooog clectrijiytc Charge was prepared having the: During the first cycle, cell potential was 3.0 volts and following Composltmn: current was 30 amperes; cell potential was 4.0 volts and Table 2 current was 55 amperes during the second cycle, and componcm wcighbpcrccm Molupcrwm cell potential was 5.0 volts and current was 100 amperes during the third hour. The procedure used for re. Z 2 covering the cathode deposits was the same as in Exam- NaIAlFJ 79 70 ple 1. Results were as follows:

Table 5 An electrolysis was then performed for three one- Product fif lfg'g i hour cycles at a temperature of l,000 i 10C. The current was 60 amps at a cell potential of 4.4 volts. After weigh of r l each one-hour electrolysis cycle. the cathode was with- 5 drawn and the deposit scraped off. The cathode was Totals 34.6 296 then replaced in the electrolyte and the electrolysis was Yield of 111B" P amp hr continued. Cathode deposits were cooled and then leached in hot 1:1 sulfuric acid to remove the adhering S5 electrolyte. Free carbon was removed from the hexa- Vacuum fuslon and cumbusnon analyses were P boride product by a float-sink separation in methylene formed on the Products of Cycles 2 and The hexaboriodide and the product was then washed in ether, dried, Product P Cycle 2 Showed 034% Carbon and and sampled oxygen, while the product of cycle 3 showed 0.48%

Total Weight of the three cathode deposits was 250g Carbon and y of which 36 grams was cerium hexaboride and 214 Spectrograph"? anaiysss of the p q recovcfed grams was adhering electrolyte. Yield of cerium hexafrom l thre? Cycles were Performed Wlth the followmg results in weight percent:

Table 6 Cycle Al Ba Ca Fe Mg Mn Si Table (i-continued C ele Al Ba (a Fe Mg Mn Si X .40 .50 .50 .20 .mo .03 .02

As may be seen from a comparison of the hexaboride product analyses with the analysis of the impure lantha nide oxide feed. a substantial degree of refining takes place during the electrolysis.

EXAMPLE 3 A number of other electrolysis runs were made with a variety of electrolyte compositions and feed materials. These experiments are set out in the following table:

In all cases, the hexaboride product was of high quality and the synthesis proccded without complication.

These examples are illustrative of the results obtained by practicing our invention. Many minor modifications of apparatus and procedure will be apparent to those familiar with the art. For example, polarity of the cell may be reversed making the crucible function as the cathode. Conductive rods of graphite may be suspended in the electrolyte to function as both anode and cathode and refractory metal rods may be used as cathodes. Multiple anodes or cathodes may be utilized instead of the single electrode system described.

We claim:

1. An electrolytic process for the preparation of lanthanide and actinide hcxaboridcs and mixtures thereof which comprises:

preparing an electrolytic bath by fusing a mixture of ingredients; those ingredients comprising a major portion of an alkali aluminum fluoride and minor portions of an alkali borate and a metal compound selected from the group consisting of lanthanide and actinide oxides, chlorides, fluorides, and mixtures thereof;

passing a direct current through the electrolyte between an anode and a cathode while maintaining the electrolyte in a molten state, and

a sodium borate and wherein the electrolyte contains a minor amount of a compound selected from the group consisting of sodium hydroxide, sodium carbonate. and mixtures thereof.

5. The process of claim 4 wherein the sodium borate is selected from the group consisting of borax. sodium metaborate. sodium orthoborate. sodium diborate, sodium tetraborate. sodium pcntaborate, and mixtures thereof.

6. The process of claim 5 wherein the electrolyte temperature is maintained within the range of 950 to l()5()C during electrolysis.

7. The process of claim 6 wherein the electrolyte contains from about 50 to mole percent cryolite.

8. The process of claim 7 wherein the metal compound is a lanthanide compound.

9. The process of claim 8 wherein the lanthanide compound is an impure mixture of lanthanide oxides.

10. The process of claim 8 wherein the lanthanide compound is selected from the group consisting of lanthanum oxide, lanthanum fluoride, and lanthanum chloride.

11. The process of claim 1 wherein the metal hexaboride containing cathode deposit is leached with acid to remove adhering electrolyte therefrom.

12. The process of claim 11 wherein the acid is hot, sulfuric acid. 

1. AN ELECTROLYTIC PROCESS FOR THE PREPARATION OF LANTHANIDE AND ACTINIDE HEXABORIDES AND MIXTURES THEREOF WHICH COMPRISES: PREPARING AN ELECTROLYTIC BATH BY USING A MIXTURE OF INGREDIENTS, THOSE INGREDIENTS COMPRISING A MAJOR PORTION OF AN ALKALI ALUMINUM FLUORIDE AND MINOR PORTIONS OF AN ALKALI BORATE AND A METAL COMPOUND SELECTED FROM THE GROUP CONSISTING OF LANTHANIDE AND ACTINIDE OXIDES, CHLORIDES, FUORIDES, AND MIXTURES THEREOF, PASSING A DIRECT CURRENT THROUGH THE ELECTROLYTE BETWEEN AN ANODE AND A CATHODE WHILE MAINTAINING THE ELECTROLYTE IN A MOLTON STATE, AND RECOVERING AS A CATHODE DEPOSIT A CRYSTALLINE METAL HEXABORIDE, SAID HEXABORIDE BEING A LATHANIDE HEXABORINE, AN ACTINIDE HEXABORIDE, OR MIXTURES THEREOF.
 2. The process of claim 1 wherein the alkali aluminum floride is cryolite.
 3. The process of claim 2 wherein the electrolyte temperature is maintained within the range of 900* to 1,100*C during electrolysis.
 4. The process of claim 3 wherein the alkali borate is a sodium borate and wherein the electrolyte contains a minor amount of a compound selected from the group consisting of sodium hydroxide, sodium carbonate, and mixtures thereof.
 5. The process of claim 4 wherein the sodium borate is selected from the group consisting of borax, sodium metaborate, sodium orthoborate, sodium diborate, sodium tetraborate, sodium pentaborate, and mixtures thereof.
 6. The process of claim 5 wherein the electrolyte temperature is maintained within the range of 950* to 1050*C during electrolysis.
 7. The process of claim 6 wherein the electrolyte contains from about 50 to 85 mole percent cryolite.
 8. The process of claim 7 wherein the metal compound is a lanthanide compound.
 9. The process of claim 8 wherein the lanthanide compound is an impure mixture of lanthanide oxides.
 10. The process of claim 8 wherein the lanthanide compound is selected from the group consisting of lanthanum oxide, lanthanum fluoride, and lanthanum chloride.
 11. The process of claim 1 wherein the metal hexaboride containing cathode deposit is leached with acid to remove adhering electrolyte therefrom.
 12. The process of claim 11 wherein the acid is hot, sulfuric acid. 